Integrated circuit

ABSTRACT

An integrated circuit including test circuitry, the test circuitry including a counter for counting clock signals and having an output for providing a control signal. The counter being arranged to have an internal state, and the counter being arranged to change the control signal on the internal state of counter reaching a predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to test circuitry and, in particular, butnot exclusively to test circuitry for incorporation in integratedcircuits.

2. Discussion of the Related Art

In all but the smallest and simplest of integrated circuits theprobability of an error or flaw is such that there is a commercialrequirement to perform a test upon the integrated circuit to determinethe integrated circuits performance prior to selling the circuit to theend user.

This testing process is automated and features as a step in theproduction of an integrated circuit. Many different automatic testingprotocols have been formulated such as functional tests, scan tests,transition fault tests, and built in self-tests (BIST).

Scan tests and transition fault tests are generally considered to beefficient and are capable of performing extensive testing on theintegrated circuit, with scan tests capable of detecting static‘stuck-at’ faults and the transition fault tests detecting‘slow-to-rise’ or ‘slow-to-fall’ faults in the circuitry.

‘stuck-at’ faults are faults where a node of the circuit is incorrectlyfabricated and retains a constant potential level. ‘slow-to-rise’ and‘slow-to-fall’ faults are faults where a node is unable to switchbetween voltage levels within the required time period.

One of the disadvantages of using automatic test equipment and scantesting is that the automatic test equipment clock frequency is limitedin speed. These frequency limitations are due to several factors, whichinclude test lead and probe length, the integrated circuit connectionpads electrical properties and the test equipment clock switchingspeeds.

Due to these limitations and the advances in high clock speed integratedcircuits it is becoming more common that automatic test equipment clocksare slower than the functional clock rate of the integrated circuit.

This limitation although not affecting the detection of ‘stuck-at’faults prevents the detection of ‘slow-to-rise’ or ‘slow-to-fall’ faultsat the operational frequency of the integrated circuit.

U.S. Pat. No. 6,430,720 describes a ring oscillator on the integratedcircuit. This oscillator is used to provide an operational test clockfrequency for testing at higher frequencies than provided by the ATE.

“High-Frequency, at-speed Scan Testing”, Lin et al, IEEE Design and Testof Computers, September-October 2003, p17-25, describes a method forperforming an at-speed scan test wherein the circuit at test is suppliedby an at-speed on board clock signal created by using PLL circuitcontrolled by the off-board clock.

The oscillator though has the disadvantage that the ring oscillator isnot the functional clock used in operating the circuitry and thus theresults produced by the test may not detect all faults exposed duringthe operation of the circuitry.

Further problems in using the solution from U.S. Pat. No. 64,300,720occur as the automatic test equipment clock and the test clock are notsynchronized. The switching between unsynchronized clock signals cancause glitches in the output clock where cycles are missed or switchedin the middle of a clock cycle.

The operating speed of the Automatic Test Equipment is dependent on thestandardization of test routines. Test routines are currently modifiedby hand in order to carry out a wider range of tests. This increasestest time which has an impact on the integrated circuit cost. This testroutine inflexibility is especially visible in variable capture counttesting.

SUMMARY OF THE INVENTION

It is one aim of the embodiments of the present invention to provideaddress or at least mitigate the problems described above.

There is provided, according to the invention, an integrated circuitcomprising test circuitry, said test circuitry comprising a counter forcounting clock signals and having an output for providing a controlsignal, said counter arranged to have an internal state, and saidcounter is arranged to change said control signal on said internal stateof counter reaching a predetermined value.

The counter may comprise a clock input, said counter being preferablyarranged to change its internal state on receiving a clock signaltransition on said clock input.

The counter may further comprise an enable input, wherein the controlsignal value may be output in dependence on an enable signal value onsaid enable input.

The counter is preferably a decrementer.

The known value may be zero.

The counter may be a linear feedback shift register (LFSR).

The counter may have an input for receiving an initial internal state,the counter preferably being arranged to be loaded with a first internalstate during a test initiation.

The counter may be arranged to receive a start signal to cause saidcounter to start counting.

The control signal value may be output in dependence on said startsignal.

When the first internal state has a second predetermined value, saidcounter may be arranged to provide said control signal and said counteris preferably further arranged to prevent the internal state fromchanging on receiving a clock signal transition on said clock input.

When said first internal state has a third predetermined value, saidcounter may be arranged to stop providing said control signal and saidcounter is preferably further arranged to prevent the internal statefrom changing on receiving a clock signal transition on said clockinput.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention and how the same maybe carried into effect, reference will now be made by way of an exampleonly to the accompanying drawings in which:

FIG. 1 shows a schematic view of a test environment featuring aschematic view of an integrated circuit and automatic test equipment;

FIG. 2 shows a schematic view of a first embodiment of the test controllogic as used in the present invention;

FIG. 3 shows a schematic view of the capture count circuitry as shown inFIG. 2;

FIG. 4 shows a flow diagram showing the method used in performing anautomatic test as applied to embodiments of the present invention;

FIG. 5 shows a timing diagram showing clock and timing signals during anautomatic test as shown in FIG. 4.

DETAILED DESCRIPTION

Reference is made to FIG. 1, which shows a first embodiment of theinvention incorporated into a test environment for testing an integratedcircuit using scan testing.

The scan test environment 51 comprises an automatic test equipment (ATE)1 which is connected to an integrated circuit to be tested 31.

The automatic test equipment has many roles in the testing of anintegrated circuit. The ATE 1 supplies to the integrated circuit 31 avariety of test signals used in the testing of the integrated circuit.Some of these signals have been defined by the Joint Testing ActionGroup (JTAG). The test data input (TDI) 41 and test data output (TDO) 43signals being test sequences or vectors.

The ATE 1 further supplies the integrated circuit 31 under test anautomatic test equipment clock TCK 45 for synchronising the testprocedure and also supplies control signals; Test mode select TMS 47,and test reset signal notTRST 49. These signals together with the TDI 41and TDO 43 signal connections are known as the test access port (TAP)interface signals.

The ATE 1 supplies the integrated circuit 31 with two further signals,TST_CAPTURE_WITH_SCAN_CLK 389, and RST_N 215 TheTST_CAPTURE_WITH_SCAN_CLK signal 389 determines whether the capture step607 is performed using the ATE controlled clock, CLK_SCANSHIFT 23, orthe on-chip generated clock, CLK_FN 22. The RST_N signal 215 performs afunctional reset on the integrated circuit.

In addition to supplying the integrated circuit 31 the TAP interfacesignals, the TST_CAPTURE_WITH_SCAN_CLK signal 389, and the RST_N signal215, the ATE 1 controls and observes all other functional inputs andoutputs. It should be realised that there are many ways in which theconnectivity between ATE 1 and integrated circuit 31 may be achieved.

The ATE 1 also calculates or knows the expected test vector from theknown functionality of the circuit under test, and compares the receivedand calculated test vectors in order to detect faults on the integratedcircuit.

The integrated circuit 31 comprises a test access port controller (TAPcontroller) 33 and a functional test integrated circuit 3.

The TAP controller 33 is configured by the ATE 1, such that the ATE 1has outputs and controls the on-chip signals: CLK_SCANSHIFT 23;TST_SCANENABLE 391; and SCAN_IN 9; and receives the SCAN_OUT signal 11.The CLK_SCANSHIFT signal 23 being the on-chip equivalent of the ATEclock TCK 45, and SCAN_IN 9 and SCAN_OUT 11 being the on-chip equivalentto TDI 41 and TDO 43 respectively. The signal TST_SCANENABLE 391 selectswhether the functional integrated circuit is in test or functional mode.The on-chip signals CLK_SCANSHIFT 23; TST_SCANENABLE 391; and SCAN_IN 9;are dependent on the received TAP interface signals Test Clock 45 TCK;47 TMS; Test data input 41 TDI; and Test data output 43 TDO. Thefunctions of known TAP controllers are defined by IEEE Standard1149.1-1990 which is hereby incorporated by reference.

The functional integrated circuit 3 as shown in FIG. 1, shows afunctional integrated circuit 3 comprising core logic 5 for processingthe functional signals; a scan path 7 which during the testing of thecircuit is connected to the ATE and receives test vectors over a firstconnection SCAN_IN 9 and outputs test vectors over a second connectionSCAN_OUT 11; a functional clock 21 designed to produce a clock signalCLK_FN 22 at the operating frequency of the integrated circuit; a clockmultiplexer 19 which is connected to both the functional clock 21 andalso the ATE clock and outputs one of the clocks to the integratedcircuit clock distribution network 17 dependent on a selection signalCLK_SELECTOR 399 (not shown). The selection signal is a function ofTST_SCANENABLE 391 and TST_CAPTURE_WITH_SCAN_CLK 389.

The use of the functional clock 21 during testing allows the circuit tobe tested at operational frequencies greater than can be supplied by theautomatic test equipment. Furthermore the use of the functional clock 21in testing as well as the operation of the integrated circuit producestest situations that are closer to the operating conditions of theintegrated circuit increasing the accuracy of the test results.

A feature of the present invention is the presence of the additionalcontrol signal TST_CAPTURE_WITH_SCAN_CLK 389. TST_CAPTURE_WITH_SCAN_CLK389 determines whether the capture step 607 is performed using the ATEcontrolled clock, CLK_SCANSHIFT 23, or the on-chip generated clock,CLK_FN 22. This differs from the current test environments whereexisting Automatic Test Pattern Generation (ATPG) software used togenerate the test patterns are not cognisant of the apparatus of thepresent invention. Therefore, if ATPG is instructed to generate patternswith TST_CAPTURE_WITH_SCAN_CLK 389 asserted, then the correct patternsare generated.

Furthermore, these test patterns can be applied to the device and thecorresponding response is correct, with only the provision that thetransition fault testing is performed at the speed of the clock,CLK_SCANSHIFT 23, supplied by ATE 1 instead of at the speed of thefunctional operating clock, CLK_FN 22.

Then, merely by deasserting TST_CAPTURE_WITH_SCAN_CLK 389, the test canbe repeated, but now the transition fault testing is performed at thespeed of the functional operating clock, CLK_FN 22. Thus the presentinvention allows transition fault testing at functional speed withoutthe need to change existing ATPG and other software.

FIG. 2 shows circuitry embodying the present invention.

The circuitry of FIG. 2 comprises a clock multiplexer 19 which has oneclock input connected to functional clock signal 22 CLK_FN from thefunctional clock 21, a second clock input connected to the automatictest equipment clock signal 23 CLK_SCANSHIFT, and a selection inputconnected to the output of a transparent latch 305. The output of theclock multiplexer 19 is connected to both the clock input of a secondtransparent latch 309 and an input of an AND gate 311.

The data input of the first transparent latch 305 is supplied a testmode signal 397 TST_MODE from a first OR gate 307. The clock input ofthe first transparent latch 305 is connected to the functional clocksignal 22 CLK_FN.

The data input of the second transparent latch 309 is supplied by theoutput of a second OR gate 321. The second OR gate 321 has a first inputconnected to an enable capture signal 393 ENABLE_CAPTURE from a capturecount unit 331, and a second input connected to the TST_SCANENABLEsignal 391 connection.

The output of the second transparent latch 309 is connected to thesecond input of the AND gate 311. The output of the AND gate 311supplies the clock signal 385 CLK_FN_AND_SCAN to the clock distributionnetwork.

The first OR gate 307 receives a scan enable signal 391 TST_SCANENABLEand a scan type signal 389 TST_CAPTURE_WITH_SCAN_CLK and provides thetest mode signal 397 TST_MODE to the first transparent latch 305.

The second transparent latch 309 and the AND gate 311 form the basis ofa gated clock buffer unit 303. The clock buffer unit in some embodimentsis also known as a clock gating element. The gated clock buffer unit 303receives the selection signal TST_SCANENABLE 391 from the automatic testequipment.

FIG. 3 shows the capture count unit 331 in more detail. The capturecount unit 331 comprises has a 3-input OR gate 401 with a first inputconnected to the clock signal 23 CLK_SCANSHIFT, a second input connectedto the test mode control signal 389 TST_CAPTURE_WITH_SCAN_CLK suppliedfrom the automatic test equipment, and an output connected to a datainput of a first flip-flop 403.

The clock input of the first flip-flop 403 is connected to the selectedclock signal CLK_SELECTED 395 from the clock multiplexer 19 and has anoutput which is connected to both the third input of the 3-input OR gate401 and also connected to the start input of a count logic unit 405.

A data output of a second flip-flop 409 is connected to the data inputof the second flip-flop 409 and also to an enable input of the countlogic unit 405. The second flip-flop 409 has a clock input connected tothe selected clock signal 395 CLK_SELECTED.

The count logic unit 405 has a further count state input connected tothe output of the counter flip-flops 407 COUNT_STATE. A count controlsignal COUNT_CTR 495 from the output of the control logic 405 isconnected to the input of the counter flip-flops 407. The counterflip-flops 407 have a clock input connected to the selected clock signalCLK_SELECTED 395.

The capture count unit 331 produces the ENABLE_CAPTURE signal. TheENABLE_CAPTURE signal 393 is input to the gated clock buffer unit 303and controls the number of pulses that are transmitted to the rest ofthe integrated circuit in the capture phase of the test.

The count logic unit 405 and counter flip-flops 407 form a counter forcounting a number of pulses received on the clock input 395 of thecounter flip flops 407. In a first embodiment of the present inventionthe counter is a simple decrementing circuit outputting a highENABLE_CAPTURE signal 393 until the state value reaches zero.

In such an embodiment the counter has an initial value n (called adivider value) and produces an ENABLE_CAPTURE 393 signal for n+1 clockperiods.

The number of clock periods required to perform a test is dependent onthe type of test and the type of integrated circuit being tested. Forexample in integrated circuits which require more than a single clockcycle to propagate the result signal to a scan chain element more than asingle cycle test will be required. For integrated circuits withseparate clocks for different parts of the integrated circuits it can benecessary to perform more than the smallest number of clock cyclesrequired by the test type. For example a circuit where one part runs attwice the speed as another part the highest speed functional clock ischosen and double the number of functional clock cycles chosen to runthe test so that the slower part is able to produce a test result.

The count logic 405 is started once the count logic receives a startsignal. The start signal SCANSHIFT_DELAYED 499 is generated when any ofthe inputs for the 3-input OR gate 401 are high and the first flip-flop403 receives a clock edge on the edge detecting clock input 423. Oncethe start signal SCANSHIFT_DELAYED 499 goes high the signal will stayhigh for the remainder of the capture phase, that is until scan data isshifted through this flip-flop during the next scan shift phase.Furthermore it can be seen that in this embodiment a high valueCLK_SCANSHIFT 23 or TST_CAPTURE_WITH_SCAN_CLK 389 will cause the 3-inputOR gate 401 to produce a high output which would start the counter assoon as the first CLK_SELECTED signal is received.

FIGS. 4 and 5 show a flow diagram of a scan test, and a timing diagramshowing examples of signals received by and generated by the integratedcircuit during a scan test respectively. The test being carried out inthis example is one which requires the use of the functional clocksignal CLK_FN 22 and therefore the automatic test equipment 1 sets theTST_CAPTURE_WITH_SCAN_CLK signal to low.

The left hand side of FIG. 4 shows the processes carried out byautomatic test equipment 1. The pathway on the right shows the effectsof the automatic test equipment 1 on the integrated circuit 3 and thetest control functional block 201 in particular.

In the first step 601 the automatic test equipment sets the signalTST_SCANENABLE 391 high in preparation for scanning in the test vector.The integrated circuit in response to this arranges the memory elementsof the integrated circuit, both test and functional memory elements, toform the scan path. As can be seen in FIG. 2 the TST_SCANENABLE signal391 is input into the first OR gate 307 and also to the second OR gate321 in the gated clock buffer 303. The TST_SCANENABLE signal 391 ispropagated through the first OR gate 307 and the first transparent latch305 to select the test clock signal CLK_SCANSHIFT 23 at the clockmultiplexer 19 to be output as the CLK_SELECTED signal 395. Furthermorethe TST_SCANENABLE signal input into the second OR gate 321 activatesthe gated clock buffer to pass the CLK_SELECTED signal 395 without theneed for an enable capture 393 signal.

In an alternative embodiment, the first OR gate 307 is placed after thetransparent latch 305; and this allows conventional scan testing even ifthe functional clock CLK_FN 22 is not running.

In the next step 603, the automatic test equipment 1, transmits on thescan path the test vector. The integrated circuit in step 603 a receivesthe test vector. The test vector not only prepares the integratedcircuit for being tested by entering the signals which will produce theresult value but is used to initialize various test scan path elements.The counter flip-flops 407 within the capture count unit can beconfigured with the divider value. The first 403 and second 409flip-flops can be configured to be switched on or off from the start ofthe capture phase.

In the next step 605, the automatic test equipment 1, prepares toperform the functional test element of the test. The TST_SCANENABLE 391signal is brought low. This can be seen in FIG. 5 at the time t₁ 501. Inthe integrated circuit the response 605 a to bringing the TST_SCANENABLEsignal low at time t₁ 501 is to switch off the scan path connecting thememory elements and return the data paths to the test pathways. The testcontrol functional block 201 also responds to the change of state of theTST_SCANENABLE signal 391. The first OR gate 307 having received a lowTST_SCANEABLE signal 391 transmits a low test mode signal TST_MODE 397to the input of the first transparent latch 305. This is passed to theclock multiplexer 19 at the next low functional clock cycle t₂ 503. Thusby the rise of the next functional clock signal the selected clocksignal CLK_SELECTED 395 from the output of the clock multiplexer 19 isthat of the functional clock signal CLK_FN 22.

The test scan enable signal TST_SCANENABLE 391, input into the gatedclock buffer 303 as the input to the second OR gate 321, switches thegated clock buffer 303 so that the gated clock buffer 303 requires anenable capture signal ENABLE_CAPTURE 393 in order to output aCLK_FN_AND_SCAN signal 385.

The TST_SCANENABLE signal change also switches the capture count unit331 from scan shifting to functional operation. At the next instance ofthe automatic test equipment clock going high CLK_SCANSHIFT, the firstflip-flop 403 receives a high input which is passed as a highSCANSHIFT_DELAYED signal 499 from the output when clocked by theCLK_SELECTED signal 395 (in other words the next functional clock edge).This can be seen in FIG. 5 at the time instance t₃ 505. The count logic405 receives the SCANSHIFT_DELAYED signal 499, actives the countflip-flops 407 using the count control signal COUNT_CTR 495 and outputsan enable capture ENABLE_CAPTURE 393 signal to the gated clock bufferunit 303. This output change can be seen in FIG. 5 at the time instancet₄ 507.

In the step 607, the capture phase, the automatic test equipment 1performs the desired number of functional clock cycles. In the exampleshown in FIG. 5 the test has been configured to carry out two clockcycles before scanning out the signal.

The second transparent latch 309 having received a high enable capturesignal ENABLE_CAPTURE 393 from the capture count unit 331, via thesecond OR gate 321, passes a high signal EN_L 387 when clocked low bythe next selected clock signal CLK_SELECTED (i.e. where the selectedclock is the functional clock CLK_FN 22). This is shown in FIG. 5 at thetime instance t₅ 509 when the EN_L signal goes high. The EN_L signal 387operates as a gating signal, in that, when the EN_L signal 387 is highthe AND gate passes the selected clock signal CLK_SELECTED 395 and whenthe EN_L 387 signal is low the selected clock signal CLK_SELECTED 395 isblocked. This can be seen in FIG. 5 in the clock pulses output on theCLK_FN_AND_SCAN signal 385 after the time instance t₆ 511. The arrowmarked 527 in FIG. 5 shows the propagation of the gating signal thatpasses from the ENABLE_CAPTURE signal 393 to the EN_L signal 387. Thisgating signal allows the clock pulses from the functional clock CLK_FN22 to be passed to the CLK_FN_AND_SCAN signal output 385 as shown by thearrows 529 and 531 in FIG. 5.

The number of clock pulses on the CLK_FN_AND_SCAN signal passed by theAND gate 311 in the test part is determined by the length of the enablecapture ENABLE_CAPTURE 393 signal as the signal is kept high. This is inturn determined by the count value stored in the counter flip-flops 407.The counter flip-flops decrement the stored count value on everyCLK_SELECTED rising edge. The stored count value is also received by thecount logic unit 405 via the count state signal COUNT_STATE 493. Whenthe stored count value reaches zero the count logic unit 405 brings theenable capture signal ENABLE_CAPTURE 393 low. In FIG. 5 this is shown attime instance t₇ 513 where on the second rising selected clock edgeCLK_SELECTED 395 (i.e. the second rising functional clock edge CLK_FN22) after t₆ 511 the enable capture signal ENABLE_CAPTURE 393 is broughtlow.

The second transparent latch 309 receives the change in the enablecapture signal ENABLE_CAPTURE 393 from the capture count unit 331 butdoes not transmit the change as the EN_L signal 387 until the clocksignal CLK_SELECTED 395 has a low level. This change is shown in FIG. 5at time instance t₈ 515. With the signal EN_L 387 low input to the ANDgate 311 no further clock pulses are passed from the selected clocksignal CLK_SELECTED 395 to the output clock signal CLK_FN_AND_SCAN 385during the remainder of the capture phase. FIG. 5 shows the two completefunctional clock high pulses 517 and 519 passed from the functionalclock CLK_FN 22 to the output clock CLK_FN_AND_SCAN 385 via the selectedclock CLK_SELECTED 395. An unbuffered series of clock pulses are alsoshown in FIG. 5 as the dashed line 521. The unbuffered clock signals 521are those generated by the gating of the functional clock signal CLK_FN22 with the enable capture signal ENABLE_CAPTURE 393. As can be seen inFIG. 5 three clock pulses are produced 521 a, 521 b, and 521 c with thefirst 521 a and last 521 c being clipped versions of the originalfunctional clock signal CLK_FN 22. Embodiments of the invention asdescribed above therefore prevent such clipped or glitched clock signalsfrom being output.

In the step 609 the automatic test equipment 1 sets the scan mode signalTST_SCANENABLE 391 high. This can be seen in FIG. 5 at time instance tg523. The integrated circuit's response to this change in theTST_SCANENABLE signal 391 is seen in FIG. 4 by the step 609 a. Theintegrated circuit switches the scan chain elements from test data pathsto the scan paths. The TST_SCANENABLE signal 391 is input into the ORgate 307 and the gated clock buffer 303. The TST_SCANENABLE signal 391is propagated through the first OR gate 307 and the first transparentlatch 305 to select the automatic test clock signal CLK_SCANSHIFT 23 atthe clock multiplexer 19 to be output as the CLK_SELECTED signal 395. Asthe first transparent latch 305 is controlled by the functional clocksignal 22 CLK_FN it is unable to pass the change in signal to the clockmultiplexer 19 until the functional clock signal 22 CLK_FN is low. Thisarrangement lowers the possibility of glitching caused by the switchingbetween unsynchronized clock signals. Furthermore the TST_SCANENABLEsignal 391 passed to the second OR gate 321 activates the gated clockbuffer 303, to pass the CLK_SELECTED signal 395 without the need for anenable capture signal 393. This can be seen in FIG. 5 at the timeinstance t₁₀ 525 where the output clock CLK_FN_AND_SCAN 385 outputs anautomatic test equipment clock pulse CLK_SCANSHIFT 23.

In the step 611 the automatic test equipment 1 reads or scans out thevalues stored in the memory elements of the integrated circuit. Theintegrated circuit in step 611 a continues to receive the automatic testequipment clock and synchronises the reading out of the values via thescan-out path using the ATE clock (CLK_SCANSHIFT) 23 as output on theCLK_FN_AND_SCAN signal 385.

Finally in step 613 the automatic test equipment 1 compares the receivedresult test vector to the calculated or known result test vector todetermine the number and possible position of faults in the integratedcircuit.

In some embodiments of the invention the OR gate 307 functions as acontrol switch with the scan type signal 389 TST_CAPTURE_WITH_SCAN_CLKdetermining whether the functional clock is to be selected during thecapture phase of the test. For example in static scan testing using asingle clock cycle there is no requirement to perform ‘slow-to’ testsand therefore the clock multiplexer 19 and the capture count unit 331are forced by a high scan type signal 389 TST_CAPTURE_WITH_SCAN_CLK toalways select and count the slower automatic test equipment clockCLK_SCANSHIFT 23.

In other embodiments of the present invention other counters as known inthe art can be used to produce a signal for a desired number of periods.For example in one embodiment a counter counting up to a maximum value,or a linear feedback shift register with the desired cycle length couldbe used.

In some embodiments of the present invention not all of the countervalues are used. If the counter logic unit 405 detects a specific valuestored in the counter flip-flops 407, then a start signal does not begina fixed length scan or transition test but triggers the capture countunit 331 to start outputting ‘infinite count’ ENABLE_CAPTURE signals393. Furthermore in the same embodiments of the present invention thecounter logic unit 405 can detect a further value stored in the counterflip-flops 407 and stops the capture count unit 331 from outputting theENABLE_CAPTURE signal 393.

Embodiments of the present invention incorporating the ‘infinite count’start and stop can be used to provide the integrated circuit with thecapacity to perform built in self-test at speed (not shown). The builtin self test (BIST) requires significantly more clock pulses than thoserequired during the capture phase of the scan tests.

In other embodiments of the present invention there may be more than onetest control unit 201 and therefore capture count unit 331 perintegrated circuit (not shown). Each capture count unit 331 can beconnected via the clock multiplexer 19 to a separate clock. In order toswitch one capture count unit on whilst preventing another from beingactivated the fourth flip-flop 409 is configured to produce a high valueto activate the counter. As previously described this configuration stepcan be carried out by the SCAN-IN signals 9 passed during the scan-inpart of the test.

In further embodiments of the present invention the gated clock buffer303 comprises a NOR gate and OR gate. The NOR gate receives the resultof the OR'ed ENABLE_CAPTURE 393 and TST_SCANENABLE 391 signals and alsothe CLK_SELECTED signal 395. The output of the NOR gate connected to afirst input of the OR gate. The second input of the OR gate is theCLK_SELECTED signal 305 with the output of the OR gate being theCLK_FN_AND_SCAN signal 385.

In other embodiments the second OR gate 321 is moved from the gatedclock buffer 303 to be before the gated clock buffer in the enablesignal path.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

1. An integrated circuit comprising test circuitry, said test circuitrycomprising a counter for counting clock signals and having an output forproviding a control signal, said counter arranged to have an internalstate, and said counter is arranged to change said control signal onsaid internal state of said counter reaching a predetermined value. 2.An integrated circuit as claimed in claim 1, wherein said countercomprises a clock input, said counter being arranged to change saidinternal state on receiving a clock signal transition on said clockinput.
 3. An integrated circuit as claimed in claim 1, wherein saidcounter further comprises an enable input, wherein the control signalvalue is output in dependence on an enable signal value on said enableinput.
 4. An integrated circuit as claimed in claim 1, wherein saidcounter is a decrementer.
 5. An integrated circuit as claimed in claim4, wherein said predetermined value is zero.
 6. An integrated circuit asclaimed in claim 1, wherein said counter is a linear feedback shiftregister (LFSR).
 7. An integrated circuit as claimed in claim 1, whereinsaid counter has an input for receiving an initial internal state, saidcounter being arranged to be loaded with a first internal state during atest initiation.
 8. An integrated circuit as claimed in claim 7, whereinwhen said first internal state has a second predetermined value, saidcounter is arranged to provide said control signal and said counter isfurther arranged to prevent the internal state from changing onreceiving a clock signal transition on said clock input.
 9. Anintegrated circuit as claimed in claim 7, wherein when said firstinternal state has a third predetermined value, said counter is arrangedto stop providing said control signal and said counter is furtherarranged to prevent the internal state from changing on receiving aclock signal transition on said clock input.
 10. An integrated circuitas claimed in claim 1, wherein said counter is arranged to receive astart signal to cause said counter to start counting.
 11. An integratedcircuit as claimed in claim 10, wherein the control signal is output independence on said start signal.
 12. A test procedure for testing anintegrated circuit, the test procedure comprising: providing anintegrated circuit having test circuitry, said test circuitry comprisinga counter; counting clock signals with the counter; outputting a controlsignal from the counter; and changing the control signal based on aninternal state of the counter reaching a predetermined value.
 13. Thetest procedure as claimed in claim 12, further comprising changing aninternal state of the counter based on the counter receiving a clocksignal transition.
 14. The test procedure as claimed in claim 12,wherein outputting the control signal from the counter comprisesoutputting the control signal value in dependence on an enable signalvalue.
 15. The test procedure as claimed in claim 12, wherein countingclock signals with the counter comprises counting backward.
 16. The testprocedure as claimed in claim 14, wherein counting backward comprisesreaching a predetermined value of zero.
 17. The test procedure asclaimed in claim 12, further comprising providing to the integratedcircuit an initial internal state during a test initiation.
 18. The testprocedure as claimed in claim 17, further comprising preventing changingthe internal state on receiving a clock signal transition when theinternal state is at a second predetermined value.
 19. The testprocedure as claimed in claim 17, further comprising stopping providingsaid control signal and preventing changing the internal state onreceiving a clock signal transition when the internal state is at athird predetermined value.
 20. The test procedure as claimed in claim12, further comprising providing to the integrated circuit a startsignal to cause said counter to start counting.
 21. The test procedureas claimed in claim 20, wherein outputting the control signal comprisesoutputting the control signal in dependence on the start signal.
 22. Anintegrated circuit comprising: test circuitry comprising a means forcounting clock signals and a means for providing a control signal, saidmeans for counting clock signals having an internal state, said meansfor providing a control signal being arranged to change said controlsignal on said internal state of said means for counting reaching apredetermined value.
 23. The integrated circuit as claimed in claim 22,wherein said means for counting clock signals comprises a clock input,said means for counting clock signals being arranged to change theinternal state on receiving a clock signal transition on said clockinput.
 24. The integrated circuit as claimed in claim 22, wherein saidmeans for counting clock signals further comprises an enable input,wherein the control signal value is output in dependence on an enablesignal value on said enable input.
 25. The integrated circuit as claimedin claim 22, wherein said means for counting clock signals comprises adecrementer.
 26. The integrated circuit as claimed in claim 25, whereinsaid predetermined value is zero.
 27. The integrated circuit as claimedin claim 22, wherein said means for counting clock signals comprises alinear feedback shift register (LFSR).
 28. The integrated circuit asclaimed in claim 22, wherein said means for counting clock signals hasan input for receiving an initial internal state, said means forcounting clock signals being arranged to be loaded with a first internalstate during a test initiation.
 29. The integrated circuit as claimed inclaim 28, wherein when said first internal state has a secondpredetermined value, said means for counting clock signals is arrangedto provide said control signal and said means for counting clock signalsis further arranged to prevent the internal state from changing onreceiving a clock signal transition on said clock input.
 30. Theintegrated circuit as claimed in claim 28, wherein when said firstinternal state has a third predetermined value, said means for countingclock signals is arranged to stop providing said control signal and saidmeans for counting clock signals is further arranged to prevent theinternal state from changing on receiving a clock signal transition onsaid clock input.
 31. The integrated circuit as claimed in claim 22,wherein said means for counting clock signals is arranged to receive astart signal to cause said means for counting clock signals to startcounting.
 32. The integrated circuit as claimed in claim 31, wherein thecontrol signal is output in dependence on said start signal.